STATS secures CNOOC offshore gas pipeline for swift repair

KINTORE, UK – STATS Group has completed its first subsea isolation project in China on a 780-km (484-mi) pipeline that supplies gas to Hong Kong.

COOEC Subsea Technology requested a temporary isolation of the Yacheng pipeline in the South China Sea. This had been damaged by a ship’s anchor and rendered unpiggable at a location around 280 km (174 mi) from shore and in water depths of 90 m (295 ft).

Following detailed engineering studies and risk assessments, the best repair solution appeared to be to remove the severely dented pipeline section and two pipeline end manifolds from the seabed onto a pipelay vessel, with the pipeline pressurized at 57 bar (827 psi).

Dale Millward, director of EPRS and subsea services at STATS, said the shutdown period for the repair work needed to be kept to a minimum, with the repair performed without depressurizing and flooding the entire pipeline.

To ensure safe recovery of both ends of the pipeline onto the pipelay vessel, STATS installed isolation tools that allowed only the sections of pipeline recovered from the seabed to be isolated.

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Due to the unpiggable defect on the pipeline, pigging of isolation plugs was not possible through the pipeline from the permanent pig launcher. STATS therefore proposed a deployment of a dual-seal BISEP isolation plug, installed at the location via a hot tap clamp fitting.

This was deployed downstream of the damaged pipeline, providing a double block and bleed isolation barrier against the pipeline gas pressure.

It allowed the pipeline to be depressurized between the rear of the BISEP and the pipeline end manifold, enabling the damaged section of pipeline to be safely cut and removed.

Millward added: “The pipeline was repaired as safely and quickly as possible, without depressurizing or flooding the entire pipeline and while ensuring that any seawater that may have entered the system was removed before the pipeline resumed operation.”